FIG. 1 illustrates an aircraft powered by a counterrotating propulsion system 2 which is of the unducted fan type. FIG. 2 illustrates the propulsion system in schematic cross-sectional form. A high-energy gas stream 6 is provided by a gas generator (not shown) and drives counterrotating turbines 9 (hatched) and 12 (not hatched) in opposite rotational directions. Counterrotating turbines 9 and 12 are directly connected to propellers 15 and 18, (also shown in FIG. 1), causing the propellers to rotate and provide thrust.
Region 21 in FIG. 2 is shown in greater detail in FIG. 3, wherein turbine stage 24A is shown, and in schematically as a hoop 22. (The ring is not actually in the form of hoop 22, as will be explained later in the specification.) The casing 24 of the turbine stage 24A experiences a wide swing in temperature. The temperature can range from an ambient temperature of, for example, 70 degrees fahrenheit during engine shutdown to 1,200 degrees fahrenheit during takeoff However, the ring 22 experiences a much smaller excursion in temperature, for example, between ambient temperature and about 400 degrees fahrenheit during flight.
In addition, the ring 22, the casing 24, and other components of the turbine stage 24A stretch because of centrifugal force. However, the ring 22 stretches significantly more because the ring carries the centrifugal load of the blades 15.
This differential in temperature swings, together with the differential in centrifugal loading, causes distance 27 between the turbine casing 24 and the ring 22 to change. The inventors have found that distance 27 can change as much as 0.090 inches. This relative change in diameters between the turbine casing 24 and the ring 22 presents a problem in fastening the ring 22 to the turbine stage 24A, inasmuch as the fastener must accommodate the change in distance 27.